fix: [PLATO-338] mui icons

- Fixing an issue with mui/material-icons breaking Netlify's deploy opennextjs/opennextjs-netlify#1174 (comment)
- Made some small changes to the image component, making the alt property more intuitive
- Fixed some faulty notFound returns in our slug page
- Hard locked the current Next version
- Updated our Node version to 18

Author: mattvanvoorst-contentful

src/components/templates/header/header.tsx

@@ -1,4 +1,4 @@
-import { Menu } from '@mui/icons-material';
+import Menu from '@mui/icons-material/Menu';
 import { AppBar, Container, IconButton, Theme, Toolbar, Box } from '@mui/material';
 import { makeStyles } from '@mui/styles';
 import { useTranslation } from 'next-i18next';